Community Antenna Television ("CATV") systems are used in a widespread manner for the transmission and distribution of television signals to end users, or subscribers. In general, CATV systems comprise a transmission subsystem and a distribution subsystem. The transmission subsystem obtains television signals associated with a plurality of CATV channels and generates a broadband CATV signal therefrom. The distribution subsystem then delivers the CATV broadband signal to television receivers located within the residences and business establishments of subscribers. The complexity and size of the distribution subsystem requires that operation and performance be periodically tested and/or monitored.
One test often performed by CATV service providers in order to pinpoint problems in the distribution subsystem is fault detection. Fault detection refers to the process of locating faults within the distribution subsystem such as breaks, shorts, discontinuities, degraded components, and improperly terminated transmission lines. Faults within the distribution subsystem are typically characterized by an impedance mismatch. In other words, the impedance of the fault is typically different than the characteristic impedance of the transmission lines of the distribution subsystem. For example, transmission lines in a CATV distribution subsystem typically have an impedance of approximately 75 ohms; however, a short on the transmission line would have an approximately zero impedance and a break would have an approximately infinite impedance.
One problem with faults in the distribution subsystem is that faults, due to their impedance mismatch characteristics, reflect signals transmitted through the distribution subsystem. As a result, beyond cutting off portions of the distribution subsystem in the case of a short or a break, faults in the distribution subsystem may also cause problems throughout the distribution subsystem due to interference from reflected signals. Therefore, it is important for CATV service providers to be able to locate faults within the network in order to cure reception problems of a single subscriber and to remove fault generated interference from the distribution subsystem as a whole.
One way of determining locations of faults within the distribution subsystem is to perform frequency domain reflectometry upon the distribution subsystem. Frequency domain reflectometry utilizes a reflectometer that applies a sweep signal to the distribution subsystem. The sweep signal is an RF signal that is swept from a start frequency to a stop frequency. If an impedance mismatch exists within the distribution subsystem, the impedance mismatch will reflect each transmitted signal back to the reflectometer at the same frequency as the transmitted signal but retarded in phase. As a result of this reflection, a standing wave is generated. The reflectometer measures the level of the standing wave at each swept frequency in order to obtain a reflected sweep response signal. The retardation of the reflected sweep response signal is such that the minimums of the reflected wave will align to 1/2 the wavelength of the impedance mismatch from the reflectometer. Due to this known relationship, the reflectometer may determine the distance from the reflectometer to the impedance mismatch.
A problem with the above frequency domain reflectometry technique is that the reflected sweep response signal typically contains harmonics. Due to these harmonics, the reflectometer may improperly indicate presence of impedance mismatches within the distribution subsystem that do not truly exist. Therefore, there is a need for a frequency domain reflectometer that is capable of removing the harmonics from the reflected sweep response signal so that the reflectometer does not improperly report presence of a truly non-existent impedance mismatch.
One frequency domain reflectometry system that corrects for harmonics is disclosed in U.S. Pat. No. 4,630,228. This system performs two sweeps on the transmission line under test in order to determine location of impedance mismatches. For the first sweep, the system internally couples a known length of reference cable to the transmission line and performs a first sweep upon the transmission line with the reference cable in order to obtain a first sweep response signal. From the first sweep response signal, the reflectometer generates a first sweep response spectrum that includes a plurality of spectral peaks that represent the frequency components of a first sweep response signal The system then decouples the reference cable from the transmission line and performs a second sweep on the transmission line without the reference cable in order to obtain a second sweep response signal. From the second sweep response signal, the reflectometer generates a second sweep response spectrum that includes a plurality of spectral peaks that represent the frequency components of a second sweep response signal.
If a spectral peak of the second sweep response spectrum is representative of an impedance mismatch at a first distance, then the spectral peak will have a corresponding spectral peak in the first sweep response spectrum that is located at the first distance plus the distance of the reference cable. However, if a spectral peak of the second sweep response is generated due to harmonics in the second sweep response signal, then the spectral peak will have a corresponding spectral peak in the first sweep response spectrum that is located at the first distance plus an integer multiple of the reference cable distance. Due to the above relationships, the reflectometer is able to discard spectral peaks that are generated due to harmonics in the sweep response signals by comparing the first sweep response spectrum with the second sweep response spectrum.
One problem with this type of reflectometer is that the reflectometer requires a lengthy reference cable in order to accurately determine which spectral peaks of a sweep response spectrum are truly due to harmonics within the sweep response signal. Alternatively, the above reflectometer may be implemented with a short reference cable; however, such a reflectometer would require the capability to accurately detect small differences in distance. Either of these two approaches adds cost to the reflectometer. The lengthy reference cable approach adds cost due to (i) the reference cable itself and (ii) the space required to house the reference cable. The short reference cable approach adds cost due to the higher quality of components required to accurately detect small differences in distance. Accordingly, there is a need for a reflectometer for locating faults within a transmission line which suppresses harmonics in a reflected sweep response signal without utilizing a reference cable.